Packet-Scheduling and Shared Channel Transmission
In wireless communication systems employing packet-scheduling, at least part of the air-interface resources are assigned dynamically to different users (mobile stations—MS). Those dynamically allocated resources are typically mapped to at least one shared data channel (SDCH). A shared data channel may for example have one of the following configurations:                One or multiple codes in a CDMA (Code Division Multiple Access) system are dynamically shared between multiple MS.        One or multiple subcarriers (subbands) in an OFDMA (Orthogonal Frequency Division Multiple Access) system are dynamically shared between multiple MS.        Combinations of the above in an OFCDMA (Orthogonal Frequency Code Division Multiplex Access) or a MC-CDMA (Multi Carrier-Code Division Multiple Access) system are dynamically shared between multiple MS.        
The main benefits of packet-scheduling are the multi-user diversity gain by time domain scheduling (TDS) and dynamic user rate adaptation.
Assuming that the channel conditions of the users change over time due to fast (and slow) fading, at a given time instant the scheduler can assign available resources (codes in case of CDMA, subcarriers/subbands in case of OFDMA) to users having good channel conditions in time domain scheduling.
Specifics of DRA and Shared Channel Transmission in OFDMA
Additionally to exploiting multi-user diversity in time domain by Time Domain Scheduling (TDS), in OFDMA multi-user diversity can also be exploited in frequency domain by Frequency Domain Scheduling (FDS). This is because the OFDM signal is in frequency domain constructed out of multiple narrowband subcarriers (typically grouped into subbands), which can be assigned dynamically to different users. By this, the frequency selective channel properties due to multi-path propagation can be exploited to schedule users on frequencies (subcarriers/subbands) on which they have a good channel quality (multi-user diversity in frequency domain).
As briefly introduced earlier in real systems the OFDM(A) physical resources (subcarriers in frequency domain and OFDM symbols in time domain) are defined in terms of subbands in frequency domain and slots, sub-frames, etc in time domain. For exemplary reasons, in the following description the following definition is used (see also 3GPP TS 36.211 V0.2.1, “Physical Channels and Modulation (Release 8)”, November 2006, available at http://www.3gpp.org and incorporated herein by reference):                A slot is defined in time domain and spans over Nsym consecutive OFDM symbols        A sub-frame is defined in time domain and spans over Nslot consecutive slots        A frame is defined in time domain and spans over Nsf consecutive sub-frames        A resource element (RE) defines the resource of one OFDM symbol in time domain and one subcarrier in frequency domain, which defines one modulation symbol        A subband is defined in frequency domain and spans over Nsc consecutive subcarriers        A physical resource block (PRB) spans over one subband and one slot and contains Nsym×Nsc resource elements        A virtual resource block (VRB) has the same size as a PRB in terms of resource elements, but has no relation to the mapping on the physical resources        
FIG. 3 shows an exemplary downlink resource grid of an OFDMA channel by means of which the structure of the resource blocks will be explained in further detail. For exemplary purposes, a frame structure as for example proposed in 3GPP TR 25.814, “Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA), (Release 7)”, version 7.1.0, September 2006 (available at http://www.3gpp.org and incorporated herein by reference) or 3GPP TS 36.211 is assumed.
Accordingly a frame may for example have a length (in the time domain) of 10 ms consisting of 10 sub-frames of 1.0 ms length. Each sub-frame may be divided in two slots each comprising a given number of NsymbDL=7 OFDM symbols in the time domain and spanning the entire downlink channel bandwidth available (i.e. all NBWDL subcarriers into which the downlink channel bandwidth is divided). Each of the OFDM symbols consists of NBWDL modulation symbols or resource elements.
As illustrated in FIG. 3, a resource block is formed by a given number of resource elements or modulation symbols in a frequency range (specified by the bandwidth of NRB subcarriers) and a given number of OFDM symbols in the time domain (or more precise the modulation symbols of the a given number of OFDM symbols in a frequency range defined by the bandwidth of NRB subcarriers). Thereby, a resource block may have the length of a sub-frame or a slot of the sub-frame in the time domain. Further, it may be assumed that a given number of resource elements in a resource block (corresponding to a given number of modulation symbols of NsymbL1/L2 OFDM symbols in a resource block) are reserved for control signaling while the remaining resource elements are used for user data.
For the 3GPP Long Term Evolution (see 3GPP TR 25.814), a 10 MHz system (normal cyclic prefix) may consist out of 600 subcarriers with a subcarrier spacing of 15 kHz. The 600 subcarriers may then be grouped into 50 subbands (a 12 adjacent subcarriers), each subband occupying a bandwidth of 180 kHz. Assuming, that a slot has a duration of 0.5 ms, a resource block (RB) spans over 180 kHz and 0.5 ms according to this example.
A number of physical channels and also reference signals will be mapped onto the physical resources (REs, PRBs). In the following, we will focus on the Shared Data CHannel (SDCH) and the L1/L2 control channels, which carry layer 1 and layer 2 control information for the data on the SDCH. For simplicity reasons the mapping of other channels and reference signals is not considered.
Typically, a physical resource block is the smallest physical allocation unit on which the SDCH is mapped. In case virtual resource blocks are defined, an SDCH might be mapped onto a virtual resource block first and a virtual resource block might then be mapped either on a single physical resource block (localized mapping) or might be distributed onto multiple physical resource blocks (distributed mapping).
In order to exploit multi-user diversity and to achieve scheduling gain in frequency domain, the data for a given user should be allocated on physical resource blocks on which the users have a good channel condition (localized mapping).
An example for a localized mapping is shown in FIG. 1, where one sub-frame spans over one slot. In this example neighboring physical resource blocks are assigned to four mobile stations (MS1 to MS4) in the time domain and frequency domain.
Alternatively, the users may be allocated in a distributed mode (DM) as shown in FIG. 2. In this configuration a user (mobile station) is allocated on multiple resource blocks, which are distributed over a range of resource blocks. In distributed mode a number of different implementation options are possible. In the example shown in FIG. 2, a pair of users (MSs 1/2 and MSs 3/4) share the same resource blocks. Several further possible exemplary implementation options may be found in 3GPP RAN WG#1 Tdoc R1-062089, “Comparison between RB-level and Sub-carrier-level Distributed Transmission for Shared Data Channel in E-UTRA Downlink”, August 2006 (available at http://www.3gpp.org and incorporated herein by reference)
It should be noted, that multiplexing of localized mode and distributed mode within a sub-frame is possible, where the amount of resources (RBs) allocated to localized mode and distributed mode may be fixed, semi-static (constant for tens/hundreds of sub-frames) or even dynamic (different from sub-frame to sub-frame).
In localized mode as well as in distributed mode in—a given sub-frame—one or multiple data blocks (which are inter alia referred to as transport-blocks) may be allocated separately to the same user (mobile station) on different resource blocks, which may or may not belong to the same service or Automatic Repeat reQuest (ARQ) process. Logically, this can be understood as allocating different users.
Link Adaptation
In mobile communication systems link adaptation is a typical measure to exploit the benefits resulting from dynamic resource allocation. One link adaptation technique is AMC (Adaptive Modulation and Coding). Here, the data-rate per data block or per scheduled user is adapted dynamically to the instantaneous channel quality of the respective allocated resource by dynamically changing the modulation and coding scheme (MCS) in response to the channel conditions. This requires may require a channel quality estimate at the transmitter for the link to the respective receiver. Typically hybrid ARQ (HARQ) techniques are employed in addition. In some configurations it may also make sense to use fast/slow power control.
L1/L2 Control Signaling
In order to inform the scheduled users about their resource allocation status, transport format and other user data related information (e.g. HARQ), Layer 1/Layer 2 (L1/L2) control signaling is transmitted on the downlink (e.g. together with the user data). Thereby, each user (or a group of users identified by a group ID) may be considered to be assigned a single L1/L2 control channel for providing L1/L2 control information to the respective user(s).
Generally, the information sent on the L1/L2 control signaling may be separated into the following two categories. Shared Control Information (SCI) carrying Cat. 1 information and Dedicated Control Information (DCI) carrying Cat. 2/3. The format of these types of control channel information has been for example specified for downlink user data transmissions in 3GPP TR 25.814:
TABLE 1FieldSizeCommentCat. 1ID (UE or group specific)[8-9]Indicates the UE (or group of UEs) for which(Resourcethe data transmission is intendedindication)Resource assignmentFFSIndicates which (virtual) resource units (andlayers in case of multi-layer transmission) theUE(s) shall demodulate.Duration of assignment2-3The duration for which the assignment is valid,could also be used to control the TTI orpersistent scheduling.Cat. 2Multi-antenna related informationFFSContent depends on the MIMO/beamforming(transportschemes selected.format)Modulation scheme2QPSK, 16QAM, 64QAM. In case of multi-layertransmission, multiple instances may berequired.Payload size6Interpretation could depend on e.g. modulationscheme and the number of assigned resourceunits (c.f. HSDPA). In case of multi-layertransmission, multiple instances may berequired.Cat. 3If asynchronousHybrid ARQ3Indicates the hybrid ARQ process the current(HARQ)process numbertransmission is addressing.Redundancy2To support incremental redundancy.versionNew data1To handle soft buffer clearing.indicatorIf synchronousRetransmission2Used to derive redundancy version (to supporthybrid ARQ issequence numberincremental redundancy) and ‘new dataadoptedindicator’ (to handle soft buffer clearing).
Similar, 3GPP TR 25.814 also suggests a L1/L2 control signaling format for uplink user data transmission:
TABLE 2FieldSizeCommentResourceID (UE or group specific)[8-9]Indicates the UE (or group of UEs) for whichassignmentthe grant is intendedResource assignmentFFSIndicates which uplink resources, localized ordistributed, the UE is allowed to use for uplinkdata transmission.Duration of assignment2-3The duration for which the assignment is valid.The use for other purposes, e.g., to controlpersistent scheduling, ‘per process’ operation,or TTI length, is FFS.TrasportTransmission parametersFFSThe uplink transmission parametersFormat (TF)(modulation scheme, payload size, MIMO-related information, etc) the UE shall use. If theUE is allowed to select (part of) the transportformat, this field sets determines an upper limitof the transport format the UE may select.
As can be recognized from Table 1 and Table 2 above, the number of control information bits is variable depending for example on the control channel information's relation to uplink or downlink user data transmissions.
Furthermore, some fields of the control channel information formats may also depend on the MIMO transmission mode of the data. For example, if data is transmitted in a special MIMO (Multiple Input Multiple Output) mode, the L1/L2 control information for this data may comprise multi-antenna related information, while this information may be omitted for data transmission without MIMO. But also for different MIMO schemes (such as Single User (SU) MIMO or Multi User (MU) MIMO) and configurations (e.g. rank, number of streams) the control channel information (prior to coding) may be different (also with respect to the number of bits).
For example, data on an allocated PRB might be transmitted to a UE using multiple codewords. In this case HARQ related information, payload size and/or modulation scheme might need to be signaled multiple times. Further, MIMO related information may include precoding related information, where the amount of required precoding information depends on the application of single user MIMO or multi user MIMO, on the rank and/or on the number of streams.
Similarly, the format (and size) of the L1/L2 control information may also depend on whether the control channel information relates to transmission of the data in a distributed or localized OFDM transmission.
In conventional systems (such as for example in UMTS High Speed Data Packet Access—HSDPA) the scheduling related control information are typically transmitted using a fixed modulation and coding scheme (MCS) level, which is known to all mobile stations within a radio cell.
Using a fixed modulation and coding scheme for L1/L2 control signaling would result in different amounts of resources that would have to be used for the L1/L2 control signaling on the physical channel resources which is however undesirable in view of UE complexity, scheduling flexibility, etc.